i am wondering if the solder-pads on the new RPi3B+ were made for an SMA connector.

or is it still for the same connector as on RPi3B/RPi0w ?
and why on earth don't they make a proper solder-mask for the connector? only one pad is able to solder, all other regions are covered with the green solder.mask, you have to scratch away...

Yes they are 'test pads only' in the official RPI stance, just like the PP pads. Otherwise they would be documented and probably populated and then an official antenna to buy as well. It would be used by people.